Known methods of preparing pellets used in molding involve the addition of compounding additives such as a filler, a stabilizer, and a pigment to polyolefin, heat-melting and kneading the mixture, and granulating it to prepare the pellets. The pellets formed in this manner are then subjected to molding processing.
However, pelletizing requires enormous energy and expensive equipment making the process industrially disadvantageous. Therefore, attempts have been made to mix particulate polyolefin with compounding additives and directly molding the resulting mixture without the above-described pelletizing step. However, mere mixing of the compounding additives such as a stabilizer, a pigment, and a filler with particulate polyolefin fails to uniformly disperse the compounding additives in polyolefin. This leads to adverse influences on the quality of resulting moldings. Of the compounding additives, a filler is generally added in a larger amount than a stabilizer or a pigment and has a finer particle size and smaller bulk density than that of particulate polyolefin. Therefore, the mere mixing with particulate polyolefin is liable to result in classification and formation of non-uniform mixture. In addition, when such a mixture is subjected to molding processing such as injection molding processing, the following problems arise:
(1) Fine filler particles are caught by the air and form dust when the mixture is thrown into a hopper or in the procedure of, for example, transferring the mixture from hopper to hopper. PA0 (2) Such a mixture is poor in feeding with a screw, which prolongs the plasticizing time, leading to a reduction in molding ability. In addition, air is trapped in pockets to the extent that deaeration becomes insufficient, resulting in silver streaks and scorch of moldings. PA0 (3) Moldings produced therefrom have poor appearance and poor mechanical properties due to the non-uniform dispersion of the filler in the moldings.
These defects become more serious as the amount of filler increases.
For the reasons described above, direct molding of the mere mixture has generally not been conducted and, under the present situation, the mixture is first pelletized, then subjected to molding processing.
The present inventors have carried out intensive investigations to obtain a polyolefin molding material comprised of particulate polyolefin and compounding additives which can be directly molded without the above-described pelletizing step. As a result of these studies the present inventors have found that a particulate polyolefin molding material comprising a particulate polyolefin particle and a layer comprising at least one compounding additive formed on the surface of the particle is excellent in moldability and gives uniform moldings.